are not a reflection of actual strength of schedule, they are popular opinion about how good a team is considered to be at that time.
I think when you beat a team who is ranked "___" you should get credit in the terms of beating ranked teams, for what they were ranked when you played them.
You already know that the outcome of the game will alter their rankings, but it doesn't change what was thought of them at the time you played them. Also, you have no control on how the other team does going forward...some teams are galvanized by a tough loss while others crumble.
In 2014 unbeaten #5 ND went to unbeaten #2 FSU and beat them until homer refs robbed them of the W...but I digress. #5 ND gave #2 FSU all that (and more) they could handle that day in Tallahassee.
FSU went on to finish their regular season 13-0, while ND lost 4 of their next 5 games. But that isn't the team FSU played that day in Tallahassee...they played the unbeaten #5 team on that day.